1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling an automatic clutch disposed between the engine and the gear transmission on a motor vehicle such as an automobile, and more particularly to a method of and an apparatus for controlling a motor vehicle clutch so that the clutch can quickly be disengaged to prevent the engine from being stopped when a brake pedal is depressed.
2. Description of the Prior Art
As is well known in the art, clutches are employed on motor vehicles for selectively transmitting engine power to driven wheels. With the recent progress of electronic technology, there has been developed an automatic control system for automatically controlling the clutch and the gear transmission on a motor vehicle. Particularly, the clutch and the gear transmission which comprises gears on parallel shafts are driven respectively by hydraulic actuators.
The automatic control system includes an automatic clutch apparatus which controls clutch engagement and disengagement by driving a clutch actuator having a cylinder and a piston for operating a clutch control member. The automatic clutch apparatus comprises an electronic control unit in the form of a microcomputer for driving the clutch actuator in dependence upon operating conditions of the motor vehicle, such as an engine rotation speed, an accelerator pedal position, and other parameters.
More specifically, the electronic control unit of the automatic clutch apparatus is arranged to compute, at all times, the extent to which the clutch is to be engaged, from a signal indicating the engine rotation speed and a signal indicating the accelerator pedal position, and hence to determine the position in which the clutch actuator is to be operated. When starting the motor vehicle, the electronic control unit determines the position in which the clutch is to be operated, from the dpeth to which the accelerator pedal has been depressed and the engine rotation speed, which depth and speed vary from time to time. The electronic control unit thus drives the clutch actuator to move the clutch control member progressively from a clutch-disengaging position through a partly engaging position to a clutch-engaging position, thereby starting to move the motor vehicle smoothly. The clutch is controlled by the electronic control unit also when the gears of the gear transmission are shifted during travel of the motor vehicle.
When the motor vehicle is at rest, the automatic clutch apparatus disengages the clutch to ensure that the engine will not be stopped but idle, since the wheels of the motor vehicle do not rotate. While the motor vehicle is running at an extremely low speed, the electronic control unit controls the clutch so as to be partly engaged to prevent the motor vehicle from being jolted due to the lack of a required torque. When the engine speed or the motor vehicle speed drops below a preset level, the clutch is disconnected to prevent the engine from being suddenly stopped when the brake pedal is depressed.
There has been proposed a clutch control system in which the deceleration of the input shaft of a gear transmission, rather than the engine speed or the motor vehicle speed, is detected, and the clutch is disengaged when the input shaft deceleration is abruptly lowered (see Japanese Laid-Open Patent Publication No. 60-8553), i.e., when there is an increased rate of the decrease in speed. According to this proposed clutch control system, as shown in FIG. 13 of the accompanying drawings, the speed of rotation of an engine 101 (or the speed, measured by a sensor 104, of rotation of an input shaft 105 of a synchromesh gear transmission 103 to which engine power is transmitted via a clutch 102) is progressively reduced, or the deceleration (i.e., the rate of reduction of the speed of rotation of the input shaft 105 per unit time) of the engine 101 is progressively increased after the brake pedal has been depressed, and the deceleration reflects the extent to which the brake pedal is depressed. Based on these characteristics, the clutch 102 is disengaged when the deceleration is higher than a preset level. Rotative drive power from the gear transmission 103 is transmitted through its output shaft 106 and a differential gear 107 to driven wheels 108 of a motor vehicle. When the driver abruptly brakes the motor vehicle, the clutch is disconnected at an earlier stage to prevent the engine from being stopped, since the input shaft 105 reaches a deceleration limit more quickly than when the motor vehicle is ordinarily braked.
The former conventional clutch control system is effective when the motor veicle is decelerated slowly, but fails to disengage the clutch 102 quickly enough when the motor vehicle is rapidly decelerated, resulting in engine stoppage. More specifically, when the accelerator pedal is released and the brake pedal is depressed relatively slowly, the engine 101 will not be stopped by disengaging the clutch 102 upon detection of the engine speed being lowered to the preset level (500 rpm). However, when the brake pedal is abruptly depressed to brake the motor vehicle quickly or when the wheel tires slip on a slippery road such as a snow-covered road and the wheels are locked by being braked, it takes a relatively long time for the electronic control unit to detect a reduction in the speed of the engine 101 and the motor vehicle down to the preset level and to get the clutch 102 to be actually operated, during which time the engine speed is quickly lowered, since the braking operation is quick, with the result that the engine 101 will be stopped before the clutch 102 is disengaged.
With the latter clutch control arrangement, as shown in FIG. 14 of the accompanying drawings, the reduction in the speed of rotation of the input shaft 105 per unit time (t.sub.0) is detected when the brake pedal is depressed, and the clutch 102 is quickly disconnected to prevent the engine 101 from being stopped when the detected reduction rate is higher than a preset level. When the motor vehicle runs on an ordinary road and is braked, the input shaft speed is lowered as shown by the curve a, and the reduction rate Ra is lower than the preset level. This means that the input shaft speed is reduced relatively gradually, and the clutch 102 is disengaged while the engine 101 is in operation. Since the input shaft speed is not lowered substantially to zero within the time period required until the clutch 102 is actually disconnected after the control system starts its operation, the engine 101 is prevented from being stopped. When the motor vehicle travels over a slippery road such as snow-covered road with a low coefficient of friction, the wheels are apt to be locked easily when braked, causing the input shaft speed to be decreased abruptly, so that its speed reduction rate or deceleration Rb becomes higher than the preset level. Because the time required for the input shaft 105 to be stopped at such deceleration Rb is shorter than the time for the clutch 102 to be completely disengaged, the engine 101 has already been stopped when the clutch 102 is disengaged.